The present invention relates to a tube for a heat exchanger in which beads for dividing passages for a medium in tubes are formed by roll forming and a method for producing the same.
It is generally known that a parallel flow type heat exchanger has a plurality of tubes laminated with fins alternately interposed therebetween and both ends of the tubes connected to communicate with header pipes, in which a medium for heat exchanging is flown to meander a plurality of times between inlet and outlet joints disposed on the header pipes so to exchange heat with the atmosphere while flowing through the tubes.
Recently, the tubes used for the aforesaid type of heat exchanger are formed by roll forming of a metallic plate material of a strip belt (hereinafter referred to as the plate) into a tube shape having beads for dividing the medium passages, and brazing its required portions.
The roll forming is performed by passing the plate material between a plurality of rotating rolls which respectively have a predetermined shape and are disposed appropriately, and beads are continuously formed on required portions of the plate in a longitudinal direction of the tube. Specifically, the beads are formed by bending the plate to have recessed shapes and have their inwardly protruded tops brazed to opposed portions of the plate so to divide the passage for the medium formed within the tube into a plurality of sections in order to improve a pressure resistance and a heat-exchanging property of the tube.
Generally, the plate in use is a brazing sheet which has a brazing material clad on its entire surface. After assembling the tubes, fins and header pipes into one body by means of a jig and the like, the required portions of the tubes and other required portions of the heat exchanger are brazed by heating the assembled body by one heating operation.
The tube formed by the roll forming as described above is provided with beads 31, 31 at predetermined intervals within a tube 30 as shown in FIG. 6 for example. And, there are disposed joint portions 32, 32 at one end of the formed tube 30 in its breadth direction, and a folding portion 33 at the opposite end in the breadth direction of the tube 30, and the beads 31 and the inside flat surface of the tube 30, and the joint portions 32, 32 are brazed mutually, and respectively.
Thus, a plurality of medium passages 34, 34 divided by the beads 31, 31 are formed within the tube 30.
In the heat exchanger tube formed by the roll forming as described above, the plate used is very thin to suit such conditions as formability, heat-exchanging property and light weight.
Therefore, where the plate is to be formed, it is necessary to suppress the plate from having an unnecessary deviation in thickness (uneven thickness of the plate in section) in order to secure a sufficient pressure resistance for the tube. Specifically, when the plate has any portion having an insufficient thickness owing to the uneven thickness, the pressure resistance of the tube is lowered. Therefore, the occurrence of the uneven thickness must be avoided.
However, the forming of the heat exchanger tube by the conventional roll forming did not take the occurrence of the uneven thickness into consideration, so that an uneven thickness was often formed when the beads were formed, and such an uneven thickness was caused conspicuously when a plurality of beads were formed in the longitudinal direction of the plate.
In the aforesaid type of heat exchanger tube, a larger stress is applied to the roots of the beads, namely the base portions of the beads, in view of the structure. The reason for this is that a stress is concentrated on the base portions of the beads due to the bent shape in the section of the plate.
Therefore, the base portions of the beads had a poor strength as compared with other portions, resulting in a disadvantage that the pressure resistance of the tube is limited.
The present invention was achieved in view of the aforesaid problems. And it is an object of the invention to provide a tube for a heat exchanger with its pressure resistance improved by reinforcing the base portions of beads and a method for producing a tube for a heat exchanger which can decrease an uneven thickness of a plate when the beads are formed.
The invention described in claim 1 is a tube for a heat exchanger which is formed to have beads for dividing passages for a medium in the tube by roll forming, wherein base portions of the beads are formed to have a thickness greater than that of a plate material for the tube.
Thus, the tube for a heat exchanger according to the invention has the plate at the base portions of the beads made thicker than that of the plate material, so that its pressure resistance can be improved further.
Specifically, the base portion of the bead requires structurally a higher stress as compared with other portions. Therefore, there were conventionally disadvantages that the base portion of the bead had a poor strength, and the pressure resistance of the tube was limited and degraded. But, according to the present invention, the plate is made thick at the base of the bead to reinforce the base of the bead, so that the pressure resistance of the tube can be improved, and the existing drawbacks can be remedied.
The invention described in claim 2 is a method for producing a tube for a heat exchanger which has beads for dividing passages for a medium in the tube formed by roll forming, wherein the roll forming comprises a first step of sequentially bending a flat metal strip plate to have depressions so to form portions-to-be-bead, which are intermediate forms of the beads, a second step of performing a width drawing of the portions-to-be-bead, and a third step of compressing the portions-to-be-bead, after the second step, in a direction of a height of the beads, wherein the portions-to-be-bead formed in the first step having a concave width wider than that of the beads formed from the portions-to-be-bead, and a height of the portions-to-be-bead after the second step is determined to be higher than the height of the beads after the forming.
As described above, according to the method for producing a tube for a heat exchanger according to the present invention, an undesired uneven thickness of the plate involved in the formation of the beads can be decreased, and the tube with the pressure resistance secured can be obtained.
Specifically, it was conventional that the plate was often made to have an uneven thickness when the beads were formed. But, according to the present invention, the portions-to-be-bead having a wider concave width than that of the formed beads are temporarily formed and then drawn, so that the plate can be suppressed from having an uneven thickness when the beads are formed, and the tube can be prevented from having a degraded strength.
And, the portions-to-be-bead are compressed after the drawing, so that the dimensional accuracy of the beads can be improved.
The invention described in claim 3 is a method for producing a tube for a heat exchanger according to claim 2, wherein the thickness of the base portions of the beads in the third step is made greater than the thickness of the plate material for the tube.
Thus, the base of the bead is made thicker than the thickness of the plate material, so that the tube having its pressure resistance further improved can be obtained.
And the increment of increase in thickness of the base of the bead can be obtained by compressing the portion-to-be-bead.
The invention described in claim 4 is a method for producing a tube for a heat exchanger according to claim 2, wherein the beads are formed in three or four, one or two at the center among them are first subjected to the first step, two beads at both ends are then subjected to the first step, and all the beads are subjected to the third step at the same time.
Thus, when three or four beads are formed, the first step is conducted on one or two beads at the center and then on two beads at both ends. Accordingly, the material can be gathered to the center portion in the process of forming the center bead or beads into a concave shape. Therefore, an uneven thickness formed when the aforesaid step is not performed, particularly a situation that the center portion is made thin because of the formation of the center bead or beads, can be prevented. And, when the material is gathered to the center by the first step, a situation that the center portion becomes thin, namely an uneven thickness, is not caused even when the third step is conducted on all the beads at the same time. Therefore, the respective beads can be formed in good balance.